The present invention relates to novel ecologically compatible, water-based, stable mullite precursor sols, methods for making the same, and mullite composites incorporating the novel compositions of the invention. The novel compositions of the invention may be used, for example, to make ceramic filters for use in industrial applications, such as gas turbines.
Gas turbine based power plants are attractive to electrical utilities because of their low cost, high efficiency, and short lead time. Unfortunately, these turbines have been limited to operation using expensive, and sometimes scarce fuels (i.e., distilled oil and natural gas). A less expensive fuel alternative is coal, and there have been efforts to develop a gas turbine system for generating electrical power that can use coal as the fuel source. When used in such applications, the coal is usually in the form of a high temperature, high pressurized gas. However, because the coal gas tends to contain particulate matter such as alkalis and sulfur, which can be harmful to the gas turbines, it must undergo pretreatment procedures before it is used in the gas turbines. In certain pretreatment procedures, ceramic filters are used to remove particulate matter from the coal gas. See, e.g., M. A. Alvin, T. E. Lippert and J. E. Lane, Assessment of Porous Ceramic Materials for Hot Gas Filtration Applications, Ceramic Bulletin, 70(9), 1991, p. 1491-1498, the disclosure of which is herein incorporated by reference in its entirety. In such applications, it is required that the ceramic filters not only have thermal, chemical, and mechanical stability, but also long term structural durability. Monolithic mullite filters can provide thermal, chemical, and mechanical stability. However, they have poor resistance to crack propagation and low tolerance to damage. Generally, an oxide continuous fiber reinforced ceramic composite is required for long term durability. It is also desirable that the ceramic filters can be processed under ecologically compatible conditions, that is, the ceramic filters can be prepared from ecologically compatible starting materials such as water-based mullite sols.
Mullite exhibits exceptional structural strength and stability in corrosive and high temperature (i.e., greater than 1000.degree. C.) environments, and thus, is a good candidate for the matrix material of ceramic composites. However, there have been problems with using mullite composites in industrial applications such as poor structural integrity of the final product due to poor density or incomplete mullitization. It is believed that these problems are due, in part, to the methodologies used in making the mullite.
Mullite has a composition of 3Al.sub.2 O.sub.3.2SiO.sub.2 and is similar to other crystalline ceramics with a high degree of covalent bonding in that relatively high temperatures are required for densification. Generally, mullite is made using conventional powder mixing techniques in which alumina and silica powder (having a particle size of approximately 1 micron) are mixed and sintered above 1600.degree. C. However, mullite prepared using these techniques tends to contain traces of other components, such as, .alpha.-Al.sub.2 O.sub.3 and .alpha.-cristobalite, which indicates that the reaction to mullite is not complete even at such high temperatures. A result of this is decreased density for compositions formed from such mullite. See, e.g., M. D. Sacks & H-W Lee, A Review of Powder Preparation Methods and Densification Procedures for Fabricating High Density Mullite, appearing in Mullite and Mullite Matrix Composites, Ceramic Transaction 6, 167-207 (S. Somiya, R. F. Davis, & J. A. Pask eds. 1990), the disclosures of which are herein incorporated by reference in their entirety. When used in industrial applications, it is desirable to have mullite compositions or compacts that have a high density because it enhances the structural integrity of the ceramic component (i.e., prevents cracks and brittleness) and enhances the useful life of the components.
It is also desirable to lower sintering temperatures for mullite (i.e., lower the mullitization temperature), and such attempts have been made by trying to reduce the effective particle size of the starting materials used for making the mullite powder. For example, colloid techniques, sol-gel techniques, and solution techniques, which generally operate based on precipitation principles, have been investigated. See, e.g., Sacks & Lee, supra. These methods typically use starting materials that have smaller particle sizes (i.e., less than 50 nm) and higher surface area (i.e., greater than 200 m.sup.2 /g), which can lead to more complete mullitization even at lower sintering temperatures. Using these methods, sintering temperatures less than about 1200.degree. C. have been obtained. See, e.g., Sacks & Lee, supra. However, despite these various methodologies employed to make mullite powders using lower temperatures, there remains the problem of producing mullite composites also having high density.
In addition, prior art techniques for making mullite powder typically use organic solvents or organic components as part of the reaction conditions, which is ecologically undesirable. There has been a move toward trying to prepare water-based mullite sols. For example, it is known that a water-based silica sol is stable in high pH (.about.8) and a water-based alumina sol is stable in low pH (.about.2). R. K. Iler, The Chemistry of Silica (John Wiley & Sons 1979); B. E. Yoldas, Ceramic Bulletin, 54(3), 1975, 286-88 & 289-290, the disclosures of which are herein incorporated by reference in their entirety. When the alumina sol and the silica sol are mixed together, a gel or precipitation is immediately formed. The sol has to stay in a solution state in order to effectively impregnate the preform so it may be processed into a mullite composite, thus, these methods are not suitable for making mullite composites.
Water-based mullite sols have also been reported in the literature for monolith and powder preparations. See, e.g., C. J. Brinker & G. W. Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, p. 216-223 (Academic Press, Inc. 1990), the disclosures of which are herein incorporated by reference in their entirety. In these applications, the sol is converted to a gel in a short time (i.e., minutes to days) so a sol having months stability is not required. However, this rapid conversion to a gel makes mullite sols prepared according to this methodology poor candidates for use in impregnating preforms, which is an essential step in making mullite composites. Water-based mullite sols having sintering temperatures of about 1000.degree. C. have been synthesized, but they contain hazardous fluoride ion and react with reinforcing preform material. Further, these mullite sols have a high exothermic phase transition from amorphous to crystalline mullite at crystallization temperatures (about 980.degree. C.) (see FIG. 1), which is an indication that mullite composites made from the sol will have poor density characteristics.
Therefore, the mullite sol is an important factor in the success of composite processing. The sol has to be stable for easy handling and processing, it should have a high solids contents for high yield, its components should have a small particle size, it should be homogeneous, it should exhibit little or no exothermic reaction during heat treatment or densification procedures before mullitization, and should be easily converted into substantially crack-free and dense mullite composites at relatively low sintering temperatures. Also, to comply with environmental issues, the sol should preferably be water based. The present invention addresses these as well as other needs by presenting aqueous mullite precursor sols, unlike the prior art mullite powders, that can be made into mullite composites. The present invention produces stable, water-based mullite precursor sols by hydrolyzing an aqueous silane solution with a base and then adding an aqueous solution of aluminum nitrate to the solution. These stable sols can then be used to prepare the novel, inter alia, substantially crack-free and dense mullite composites of the invention.